Conventionally, an oscillator is used to time (synchronize) operation of circuits in an electronic device or the like. The oscillator capable of accurately outputting an electric signal used as a reference of operation is an indispensable device for such an electronic device. A crystal oscillator using a crystal resonance unit is one example of such an oscillator. However, the crystal oscillator involves problems such as: difficulty in achieving miniaturization, unsuitability for integration, numerous experimental production steps, poor yields, and required long time for delivery. Hence, in recent years, an oscillator that uses micro electro-mechanical systems (MEMS: Micro Electro-Mechanical Systems) prepared through a semiconductor process with silicon or the like is drawing attention as a device that replaces the crystal oscillator.
The micro electro-mechanical oscillator (referred to as the “MEMS oscillator”, hereinafter,) includes a feedback type oscillator circuit structured with an amplifier circuit and a MEMS resonator. The MEMS resonator exhibits a significantly improved electrical pass characteristic between the input and output electrodes, as to only an electric signal of a frequency near a specific frequency, i.e., the resonance frequency of the MEMS vibrator (natural frequency of the vibrator). In the MEMS oscillator, using such a characteristic of the MEMS vibrator, an oscillating state is created by feeding back an electric signal of a resonance frequency included in the output from the amplifier circuit to the amplifier circuit. Then, the MEMS oscillator outputs the electric signal having been output from the amplifier circuit in the oscillating state as an oscillator signal. Accordingly, the frequency of the oscillator signal output from the MEMS oscillator is determined based on the resonance frequency of the MEMS vibrator.
It is known that the resonance frequency of the MEMS resonator has temperature dependence. The MEMS resonator is generally formed with silicon or the like, and due to the temperature characteristic of silicon, its resonance frequency has a temperature characteristic of about −20 [ppm/degree Celsius]. For example, when the operating temperature changes by 100 degrees Celsius from −20 to +80 degrees Celsius, the resonance frequency changes by about 2000 [ppm]. Accordingly, in accordance with a change in the operating temperature of the MEMS resonator, the frequency of the oscillator signal similarly changes. Therefore, with a conventional MEMS oscillator, a temperature sensor is disposed near the MEMS resonator, and based on the temperature measured by the temperature sensor, any frequency fluctuation in an oscillator signal due to a temperature dependence of the resonance frequency of the MEMS resonator is compensated for. Thus, irrespective of the temperature, electric signals of a constant frequency are output.
FIG. 21 is a block diagram of a conventional MEMS oscillator. The conventional MEMS oscillator 300 includes an oscillator unit 301 that outputs an original oscillator signal; and a corrector unit 302 that corrects a frequency of the original oscillator signal and outputs it as an output signal having a desired frequency (see PATENT LITERATURE 1).
In the oscillator unit 301, a feedback type oscillator circuit is structured with an amplifier 312 and a MEMS resonator 313, and the output from the amplifier 312 is taken out as an original oscillator signal, and is input into the corrector unit 302.
When the resonance frequency of the MEMS resonator 313 fluctuates due to a change in the temperature or the like, the frequency of the original oscillator signal similarly fluctuates. In the MEMS oscillator 300, by the corrector unit 302 compensating for a fluctuation in the frequency of the original oscillator signal, the frequency of the output signal is maintained constant.
The corrector unit 302 includes a PLL (Phase-Locked Loop) circuit 321, a division ratio controller unit 322 controlling the division ratio of the frequency divider (not shown) disposed at the feedback of the PLL circuit 321, and a temperature sensor 1101.
Based on an input from the temperature sensor 1101, the division ratio controller unit 322 adjusts the division ratio of the frequency divider (not shown) disposed at the feedback of the PLL 321, such that the frequency of the output signal that the PLL 321 outputs attains a desired value. More specifically, the division ratio controller unit 322 determines the division ratio of the frequency divider (not shown) provided at the feedback of the PLL 321 from the temperature characteristic of the known resonance frequency of the MEMS resonator 313, the input from the temperature sensor 1101, and a preset frequency of the output signal.
FIG. 22 is a side cross-sectional view of the MEMS resonator 300 described above. As shown in FIG. 22, the MEMS resonator 313 is packaged such that the surrounding of the vibrator is maintained under vacuum, in order not for the air or the like to affect the mechanical vibration of the vibrator. The MEMS resonator 313 having such a structure is formed as a second chip 1302 which is separate from a first chip 1301 where the amplifier unit 312 and the corrector unit 302 are formed. The temperature sensor 1101 is formed near the MEMS resonator 313 in the first chip 1301.
Then, the first chip 1301 and the second chip 1302 are connected to each other by a metal wire 806 connecting between a pad 604 connected to a wiring 803 extending from the exterior surface of the second chip 1302 to the front layer of the circuit and a pad 805 connected to the first chip 1301, and installed in a cascade manner.
As described above, the surrounding of the vibrator of the MEMS resonator 313 is in a vacuum state. Therefore, the thermal conductivity between the vibrator and the outside is low. Therefore, a difference arises between the temporal fluctuation in the temperature measured by the temperature sensor 1101 of the first chip 1301 and the temporal fluctuation in the actual temperature of the vibrator of the MEMS resonator 313.
FIG. 23 is a graph schematically showing an example of the temporal fluctuation of the temperature measured by the temperature sensor 1101 and that of the actual temperature of the vibrator in the MEMS resonator 313. When a temperature 901 measured by the temperature sensor 1101 fluctuates as shown in FIG. 23, an actual temperature 902 of the vibrator fluctuates to follow the temperature 901, while slightly lagging behind the temperature 901. Therefore, the temperature 901 measured by the temperature sensor 1101 agrees with the actual temperature 902 of the vibrator only at a time period D903 and very limited time points such as time points T904, T905 and T906, and they do not agree with each other at the other time points. In other words, with the structure of the conventional MEMS oscillator 300, it is difficult to accurately compensate for the temperature dependence of the resonance frequency of the MEMS resonator 313 in real time based on the actual temperature of the vibrator, such that output signals of a frequency that accurately agrees with a desired frequency are always output.